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Explore the cutting-edge Shadows system, a blend of synthetic aperture sonar and forward-looking technology implemented in shallow water surveys in Portsmouth during spring 2008. This comprehensive study covers the system architecture, processing techniques, and detailed results obtained in the complex and dynamic marine environment. Discover the innovative methods used for real-time georeferenced image acquisition and processing. Gain insights into the complex image information analysis, including spatial Fourier transforms and advanced data handling techniques. The study also outlines future research avenues for improving survey efficiency and data quality.
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Shadows, combination of synthetic aperture sonar and forward looking gap-filler. Results in shallow water in Portsmouth during spring 2008. Portsmouth, Shallow Survey 2008, oct. 21-24, 2008, F. Jean
Contents • Introduction • Presentation of the Shadows system • Side Scan SAS Processing • Forward looking sonar Processing • Results in shallow water • Complex Image Informations • Further works
Introduction • Side scan Sonar : • Grazing angle • Echoes & shadows • Gap • Settings for Shadows : • Maximal range 300m ( 600m swath ) • Resolution 15cm • Filling the gap with comparable resolution
Shadows system • Shadows architecture • Sides • 2 m long, 24 receivers • 3 transmitters • F = 100kHz • B = 30 kHz • Front • 70cm wide, 48 receivers • 4 transmitters • F = 300kHz • B = 40kHz Real time georeferenced images PC algorithm PC acquisition ins
Side Scan SAS • Synthetic Aperture Sonar : • Sum of successive pings • Length (~number of pings) adapted to range • constant resolution along the swath • Algorithm Steps • INS Motion Compensation • DPCA Motion Compensation • Fusion and Real time beam forming (FFBP)
Side Scan SAS • INS motion compensation • Highly accurate and synchronised INS • Aided by a DVL -> limited linear drift • Realtime georeferencing • DPCA motion compensation • Using correlation between signals • Real time georeferenced mosaic
F1 F2 F3 F4 Forward looking sonar SAS data SAS data • Sectorized Emission Principle: • 4 different footprints • Ambiguous reception • Real Time: • Incoherent Integration of several pings • Post Processing: • movie of a target.
Fusion • Geo-referenced map • Keeping Continuous textures • No deformation straight line on the sea-bottom -> straight line on the image
Results in shallow water Portsmouth, spring 2008
Results in shallow water 20m x 20m image 30 meter water depth Range : approx. 70m 50m x 50m image 30 meter water depth Range : 20m to 100m
Complex Image Informations 2D • Spatial Fourier transform on amplitude 1D Space (m) Time (s) Frequency (Hz) Spatial frequency (m-1)
Complex Image Informations • Spatial Fourier transform on complex data Amplitude phase Amplitude (constant) phase Frequency Frequency • -> All information is in the Phase
Complex Image Informations • FFT of amplitude • Real Data : 1mx1m structure • Amplitude • FFT of complex • Phase • 8cm : alongtrackresolution (SAS) • 3cm : acrosstrackresolution • (30kHz Bandwidth)
Lowfrequency pixels Complex Image Informations • Partial Spectrum Images High frequency pixels
Complex Image Informations • Filtering using phase of image • Hermitian filter : Enhancing the ‘white pixels’
Further Works • Side scan SAS • Despeckling images • Work on different frequencies (higher or lower) • Interferometry • Forward looking Sonar • Increase fusion quality between sectors • Coherent integration
